Alkali metal chlorate, and particularly sodium chlorate, is an important chemical in the cellulose industry, where it is used as a raw material in the production of chlorine dioxide, which is an important bleaching chemical for cellulose fibers. Alkali metal chlorate is produced by the electrolysis of an electrolyte containing alkali metal chloride according to the overall formula: EQU MeCl+3 H.sub.2 O.fwdarw.MeClO.sub.3 +3 H.sub.2 (me=alkali metal)
The process is cyclic; where in a first step, the chloride electrolyte is brought to an electrolyzer for the formation of hypochlorite, whereupon the solution is brought further to reaction vessels for further reaction to chlorate. Subsequently, chlorate formed is separated by crystallization. The crystallization of chlorate can be brought about by evaporation or cooling. Evaporation means that the water is evaporated and condensed in a separate step, either indirectly in a heat exchanger or, more frequently, directly in the cooling water. Cooling crystallization means that the temperature is lowered to such an extent, that the chlorate electrolyte becomes saturated with chlorate whereby crystals precipitate.
In the cyclic chlorate process, the pH is regulated in several positions within the range 5.5-12, to optimize the process conditions in each unit operation. Thus, a weakly acidic or neutral pH is used in the electrolyzer and reaction vessels to promote the formation of hypochlorite, while the pH in the crystallizer is alkaline to avoid the reaction of hypochlorite to chlorine instead of to chlorate and also to reduce the risk of corrosion.
Normally, hydrogen chloride is used to lower the pH, but also, chlorine is used completely or partly. Normally, alkali metal hydroxide is used to make the solutions alkaline. Hydrogen chloride and alkali metal hydroxide are added as aqueous solutions. Commercially available technical solutions of hydrochloric acid and alkali metal hydroxide contain impurities, which even at low contents are unfavorable for the chlorate electrolysis.
A chloride electrolyte to be electrolyzed in a chlorate cell, must not contain high contents of impurities. Thus, Ca.sup.2+, Mg.sup.2+ and SO.sub.4.sup.2- cause depositions on the cathodes and thereby a higher operating voltage and energy costs, while heavy metals decompose the hypochlorite formed to chloride and oxygen, instead of, as desired, to chlorate. To avoid these drawbacks, the chloride electrolyte is normally purified, which most simply takes place already in the preparation of brine by dissolution of the technical salt. In this part of the process, the flow is small and chlorine compounds such as molecular chlorine and chlorate have not yet been formed. The impurities may also be removed later in the process before the chloride electrolyte is introduced into the electrolyzers. The purification can be brought about by the addition of chemicals containing CO.sub.3.sup.2-, OH.sup.- and Ba.sup.2+ for the precipitation of e.g. calcium carbonate, magnesium hydroxide and barium sulphate and by letting the brine or chloride electrolyte pass ion-exchange resins where additional Ca.sup.2+, Mg.sup.2+ and also Ba.sup.2+ are removed. Suitably, alkali metal hydroxide is used in the purification of brine and the regeneration of ion-exchange resin.
Normally, a chloride electrolyte to be electrolyzed in a chlor-alkali process, must also be purified from impurities. This is especially valid for membrane cells, where magnesium and calcium hydroxide can be precipitated in the membrane and cause increased operating voltage and reduced current yield, if not substantial purification measures are brought about. In this connection, the chloride electrolyte or brine is treated in a similar way as the solution intended for the chlorate electrolysis, i.e. by precipitation with chemicals and separation followed, by ion exchange. Suitably, alkali metal hydroxide is also used here. In this case, the chlorine content in the recirculating chloride electrolyte must be reduced down to the ppm level, since presently available ion-exchange resins in the final purification step are not resistent to free chlorine. In this connection, a vacuum is used in one or more steps, as well as adsorption on active carbon and/or chemical reaction with e.g. hydrogen peroxide.
According to CA 1,178,239, a process for the production of sodium chlorate is combined with a membrane cell for the production of chlor-alkali. In this case, the object is to simultaneously produce sodium hydroxide, chlorine, hydrochloric acid and sodium chlorate, the chemicals needed in the cellulose production. In the chlorate electrolyzer and membrane cell, aqueous chloride electrolyte is electrolyzed, which electrolyte is obtained by the addition of sodium chloride to the depleted chloride electrolyte from the anode compartment of the membrane cell. The concentrated chloride electrolyte is purified in two steps, where the content of Ca.sup.2+, Mg.sup.2+ and SO.sub.4.sup.2- are reduced in the first step by precipitation. The contents of Ca.sup.2+, Mg.sup.2+, Ba.sup.2+ and the heavy metals are further reduced in a second step, by contacting the solution with an ion-exchange resin. The method for precipitation of chlorate is not evident from the patent. Use of the produced sodium hydroxide in the production of sodium chlorate, is not mentioned either in the patent.
According to U.S. Pat. No. 3,897,320, chlorine and alkali metal hydroxide are produced in a membrane cell by electrolysis of an aqueous alkali metal chloride solution. The chlorine and chlorate-containing anolyte in the membrane cell, is transferred to a chlorate cell for further electrolysis to chlorate, which is precipitated in a crystallizer. The remaining mother liquor is returned to the membrane cell by way of the arrangement for the production of fresh alkali metal chloride solution or directly to the chlorate cell. It is not evident from the patent, if the precipitation of chlorate takes place by cooling or evaporation with direct or indirect condenser. In this combined process, conventionally purified water and alkali metal chloride are used without any special purification step, which makes it necessary to withdraw contaminated products such as chlorate and alkali metal hydroxide, so that the contents of impurities can be controlled. Thus, this process is not useful when the process is closed to a high extent or when pure products are required. The alkali metal hydroxide produced is preferably used in cooking and bleaching of groundwood pulp.
Thus, various methods have been proposed to keep the concentration of impurities in the chlorate process at an acceptable level. Common to these methods is either expensive purification of the raw material or discharge of unwanted substances from the process after that they have been allowed to contaminate the chloride electrolyte and electrolyte of various concentrations. The discharge occurs either by one or more purification steps in the process or by the impurities accompanying the products. To avoid accumulation of impurities in the chlorate electrolysis and with that an accompanying requirement for purification measures, is what this invention aims at solving.